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Marine Ecology Progress Series 491:165

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Marine Ecology Progress Series 491:165 Vol. 491: 165–175, 2013 MARINE ECOLOGY PROGRESS SERIES Published October 2 doi: 10.3354/meps10450 Mar Ecol Prog Ser Salp-falls in the Tasman Sea: a major food input to deep-sea benthos Natasha Henschke1,2,*, David A. Bowden3, Jason D. Everett1,2,4, Sebastian P. Holmes5,6, Rudy J. Kloser7, Raymond W. Lee8, Iain M. Suthers1,2 1Evolution and Ecology Research Centre, University of New South Wales, Sydney, New South Wales 2052, Australia 2Sydney Institute of Marine Science, Building 22, Chowder Bay Road, Mosman, New South Wales 2088, Australia 3National Institute of Water and Atmospheric Research Ltd. (NIWA), 301 Evans Bay Parade, Greta Point, Wellington 6021, New Zealand 4Plant Functional Biology and Climate Change Cluster, Faculty of Science, University of Technology Sydney, PO Box 123 Broadway, New South Wales 2007, Australia 5School of Biological Sciences, University of Sydney, Sydney, New South Wales 2006, Australia 6Water & Wildlife Ecology Group (WWEG), School of Science & Health, University of Western Sydney (UWS), Penrith, New South Wales 1797, Australia 7Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine Laboratories, PO Box 1538, Hobart, Tasmania 7001, Australia 8School of Biological Sciences, Washington State University, PO Box 644236, Pullman, Washington 99164-4236, USA ABSTRACT: Large, fast-sinking carcasses (food-falls) are an important source of nutrition to deep- sea benthic communities. In 2007 and 2009, mass depositions of the salp Thetys vagina were observed on the Tasman Sea floor between 200 and 2500 m depth, where benthic crustaceans were observed feeding on them. Analysis of a long-term (1981 to 2011) trawl survey database determined that salp biomass (wet weight, WW) in the eastern Tasman Sea regularly exceeds 100 t km−3 yr−1, with biomasses as high as 734 t km−3 recorded in a single trawl. With fast sinking rates, salp fluxes to the seafloor occur year-round. Salps, like jellyfish, have been considered to be of low nutritional value; however, biochemical analyses revealed that T. vagina has a carbon (31% dry weight, DW) and energy (11.00 kJ g−1 DW) content more similar to that of phytoplankton blooms, copepods and fish than to that of jellyfish, with which they are often grouped. The depo- sition of the mean yearly biomass (4.81 t km−2 WW) of salps recorded from the trawl database in the Tasman Sea represents a 330% increase to the carbon input normally estimated for this region. Given their abundance, rapid export to the seabed and high nutritional value, salp car- casses are likely to be a significant input of carbon to benthic food webs, which, until now, has been largely overlooked. KEY WORDS: Benthic communities · Gelatinous zooplankton · Carbon cycling · Fluxes · Salp-fall · Jelly-fall Resale or republication not permitted without written consent of the publisher INTRODUCTION including phytoplankton blooms, other plant or algal matter, zooplankton faecal pellets and carcasses of Food-limited deep-sea benthic ecosystems rely on larger fauna are major contributors of organic matter depositions of organic matter from the euphotic zone to the sea floor (Rowe & Staresinic 1979, Smith et al. (Gooday 2002). Concentrated pulses of particulate 2008). Despite the majority of particles being small organic matter (POM) derived from differing sources (<5 mm) (Alldredge & Silver 1988), these pulses are *Email: [email protected] © Inter-Research 2013 · www.int-res.com 166 Mar Ecol Prog Ser 491: 165–175, 2013 an important source of nutrition for deep-sea benthic al. 1989, Lyle & Smith 1997, Gili et al. 2006), they are communities, promoting both species richness and still generally thought to be of low nutritional value abundance (Butman et al. 1995). Benthic ecosystem (Moline et al. 2004). Therefore, to determine whether functions are also positively related to increasing salp carcasses can positively contribute to the ben- POM supply, including sediment community respira- thic ecosystem, it is necessary to identify the quality tion rates and organic matter remineralisation (Witte of food they provide. & Pfannkuche 2000, Smith et al. 2008, Sweetman & In particular, we sought to (1) assess the frequency Witte 2008). and abundance of salp swarms in the Tasman Sea Large, fast-sinking particles, such as carcasses, and eastern New Zealand over 30 yr, (2) quantify the provide food-fall events that augment the nutritional biomass and relative abundance of Thetys vagina ecology of deep-sea benthic communities (Rowe & carcasses on the sea floor, and (3) compare the ener- Staresinic 1979, Stockton & DeLaca 1982, Smith & getic input and the biochemical composition of T. Baco 2003). The ‘gelatinous pathway’ (Billett et al. vagina carcasses with other gelatinous zooplankton. 2006, Lebrato et al. 2012) was first discovered by Moseley (1880) and illustrates the potential for sink- ing carcasses of gelatinous organisms to contribute a MATERIALS AND METHODS large flux of organic matter to the benthic environ- ment. Because of their swarming nature, depositions Study region of gelatinous carcasses generally accumulate in high densities to the benthic environment in areas under- Long-term trawl surveys and 2 benthic sampling lying large and persistent gelatinous populations cruises were conducted in the southern Tasman Sea (Billett et al. 2006, Lebrato & Jones 2009). For ex - and Pacific Ocean east of New Zealand (Fig. 1A). For ample, following swarms in surface waters (Wiebe et the first benthic study on board the RV ‘Tangaroa’ in al. 1979, Grassle & Morse-Porteous 1987), dense con- June 2007 (TAN0707), sampling was carried out on centrations of salp carcasses were observed nearby the Challenger Plateau, a large submarine plateau on the seafloor in the outer Hudson Canyon (3240 m) extending from the west coast of central New Zea - in 1975 and 1986 (Cacchione et al. 1978). Similarly, land and considered to be a region of low pelagic pelagic cnidarian deposits (jelly-falls) have been re- productivity (Wood 1991). In October 2009, on the corded on the sea floor off Oman (Billett et al. 2006), second benthic study on board the RV ‘Southern Sur- in the Sea of Japan (Yamamoto et al. 2008) and in a veyor’ (SS03/2009), sampling occurred off southeast- Norwegian fjord (Sweetman & Chapman 2011), while ern Australia in Bass Canyon, one of the largest sub- pyrosome carcasses have been observed on the marine canyons in the world (Mitchell et al. 2007). Madeira Abyssal Plain (Roe et al. 1990) and on the seafloor off the Ivory Coast (Lebrato & Jones 2009). During 2 benthic sampling research voyages, we Trawl data analysis observed mass depositions of the large salp Thetys vagina on the Tasman Sea floor, prompting an exam- Trawl data were available from 2 sources: a long- ination into their subsequent fate and the nutritional term data series (30 yr) from the New Zealand fish- value provided by the carcasses to the deep-sea ben- eries research trawl database and pelagic trawls in thic communities. T. vagina reaches up to 306 mm in the Tasman Sea over 3 yr. Salp and pyrosome bio- size (Nakamura & Yount 1958) and has a distribution mass was obtained from analysis of the New Zealand spanning the top 200 m (Thompson 1948, Iguchi & fisheries database (stock assessment, research and Kidokoro 2006) of sub-tropical and temperate waters observer-monitored commercial trawls) from 1981 to of the Mediterranean Sea and the Atlantic, Indian 2011 (n = 2044; see Fig 1A for sampling locations). As and Pacific oceans (Berrill 1950). Salp carcasses can the majority of data was opportunistically sampled, potentially sink at rates of up to 1700 m d−1 (Lebrato sampling periods within a year are variable but on et al. 2013), suggesting that little, if any, decomposi- average include every month per year. Trawls (mid- tion occurs during descent, and mass depositions of water or benthic) were towed at a mean depth of salp carcasses may represent an important and sub- 563.17 ± 342.40 m, ranging from 33 to 2532 m. Where stantial food-fall event for the benthic ecosystem. possible, recorded trawl dimensions and tow lengths Although several reports indicate that gelatinous were used. If details of trawl size or tow distance organisms such as salps are important to the diet of were not available, a standard averaged value calcu- some marine organisms (e.g. Duggins 1981, Clark et lated from all trawls was used (headline height = 8 m, Henschke et al.: Salp carcasses as food-fall 167 30° S A Australia Sydney 35° Tasman Sea Auckland New Zealand 40° Wellington Hobart 45° 1981−2011 Salp and pyrosome trawls 2008−2011 T. vagina trawls 2007 Challenger Plateau T. vagina deposition 2009 Bass Canyon T. vagina deposition 50° 140°E 145° 150° 155° 160° 165° 170° 175° 180° 00 38° 36° 2 S B S C Salps per 1000m −2000 No count Australia 1 −1000 −200 10 38° −600 100 −1000 −2000 − −1000 38.5° 2000 −600 −600 Salps per 1000m 2 − 40° 1000 50 250 500 New 200 − Zealand 39° 42° 147°E 148° 149° 150° 166°E 168° 170° 172° Fig. 1. (A) Survey area in the southern Tasman Sea and southwestern Pacific Ocean east of New Zealand showing trawl sta- tions and benthic sampling stations. (B,C) Density distribution based on video footage/camera stills of Thetys vagina (ind. 1000 m−2) at different stations (B) in Bass Canyon and (C) on the Challenger Plateau. Depth contours are displayed in metres wing distance = 30 m, tow distance = 4.4 km, tow conservative than data obtained from the trans- speed = 6.5 km h−1). Individuals were not classified Tasman pelagic trawls. All biomass estimates are into species. Thetys vagina biomass was obtained represented in wet weight (WW). from 3 trans-Tasman cruises in 2008, 2009 and 2011 (n = 12). Depth-stratified midwater tows with a pelagic trawl were made at 200 m intervals to a max- Benthic sample collection and analysis imum depth of 1000 m from the surface, with equal 20 min tows at 6.5 km h−1.
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